Optical waveguides – Polarization without modulation
Patent
1992-11-24
1994-04-26
Lee, John D.
Optical waveguides
Polarization without modulation
385 31, 385 24, G02B 626
Patent
active
053074289
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical apparatus of particular, but not exclusive, application to routing pulsed optical signals.
2. Related Art
In time division multiplexed communication systems there is a need to perform operations on selected bits of data, which operations include removing and inserting bits in selected time slots from, and into, a received optical data stream. The present invention seeks to provide an optical routing device which can perform such operations in the optical regime, i.e., without the need to convert the data stream to an equivalent electrical data stream upon which the operations are performed.
SUMMARY OF THE INVENTION
The present invention provides an optical apparatus comprising a first optical coupling means having first and second pairs of optical communication ports, in which substantially equal portions of an optical signal received at a port of one pair of ports are coupled to each port of the other pair of ports; an optical waveguide means optically coupling the second pair of ports such that an optical signal at a first wavelength leaving either one of the second pair of ports in a first polarization state arrives at the other port of that pair in a second polarization state substantially orthogonal to the first polarization state; a second optical coupling means for coupling an optical signal at a second wavelength to the optical waveguide so as to propagate along at least a portion of it in one direction only; and first and second polarizing beam splitters, each of which is optically coupled to a respective one of the first pair of ports.
The optical waveguide forms a waveguide loop coupling the second pair of ports.
In this application, by material having a non-linear refractive index it is meant the refractive index of the material varies with the intensity of the transmitted signal. Typically the refractive index n is given by the formula n+n.sub.0 .+-.n.sub.2 .vertline.E.vertline..sup.2 where n.sub.0 is the linear refractive index, n.sub.2 is the Kerr coefficient and .vertline.E.vertline..sup.2 the intensity of the transmitted signal.
The polarisation controller is adjusted so that the polarization state of a signal portion when it arrives at the first optical coupling means, after propagating around the optical waveguide loop, is orthogonal to the polarization state it had when it left the first optical coupling means before propagating around the loop whichever way round the loop it propagates.
The first optical coupling means and the optical waveguide loop form a Sagnac anti-resonant interferometer which acts (in the absence of a signal at the second wavelength) as a Mach-Zehnder interferometer to optical signals at the first wavelength. Because the counter-propagating pulses have different polarization states in the loop between the polarization controllers, they will in general experience different optical path lengths. If the counter-propagating portions arrive at the coupler in phase, a signal entering the coupler at a first port will be reflected, i.e. it will exit the coupler at the same port, if .pi. radians out of phase it will be transmitted, i.e. it will exit the coupler at the other port.
When an optical signal at the second wavelength of appropriate intensity (a control signal) is coupled to the optical waveguide so as to propagate along it in one direction only, the portion of a co-propagating optical signal can be made to experience a phase shift relative to a portion propagating without the control signal round the loop in the other direction. This is additional to any phase difference which occurs in the absence of the control signal. Thus, an optical signal coupled to an input port of the coupler will be transmitted or reflected rather than vice-versa in the presence of the control signal.
The first optical coupling means is conveniently a dichroic optical coupler coupling most of an optical signal received at one port of the first pair of ports at the second wavelength to one port of
REFERENCES:
patent: 4468090 (1984-08-01), Ulrich et al.
patent: 4469397 (1984-09-01), Shaw et al.
patent: 4720162 (1988-01-01), Mochizuki et al.
patent: 5050183 (1991-09-01), Duling, III
Optics Letters, vol. 14, No. 14, Jul. 15, 1989, Optical Society of America, Blow et al: "Experimental demonstration of optical soliton switching in an all-fiber nonlinear Sagnac interferometer," pp. 754-756.
Applied Physics Letters, vol. 55, No. 1, Jul. 3, 1989, (New York, US), Farries et al: "Optical fiber switch employing a Sagnac interferometer," pp. 25-26.
Blow Keith J.
Doran Nicholas J.
Nelson Brian P.
British Telecommunications public limited company
Heartney Phan T.
Lee John D.
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